Hello,
the IOS requirements for SDM version 2.1 on the 1751 router are:
12.2(13)ZH or later
12.3(2)XA or later (SDM does not support Cisco IOS release 12.3(2)XF)
12.2(13)T3 or later
12.3(2)T or later
12.3(1)M or later
12.2(15)ZJ3 (not available for the 1710 or 1721)
For SDM 1.0, the requirements are:
12.2(13)ZH, or later
12.2(13)T3 or later
12.3(1)M or later
12.2(11)T6 not supported
I guess you need to ask a reseller/partner for the upgraded IOS, not sure what they will charge you fir that...
HTH,
GP